How fluid infiltrates dry crustal rocks during progressive eclogitization and shear zone formation: insights from H2O contents in nominally anhydrous minerals

Abstract

Granulites from Holsnøy (Bergen Arcs, Norway) maintained a metastable state until fluid infiltration triggered the kinetically delayed eclogitization. Interconnected hydrous eclogite-facies shear zones are surrounded by unreacted granulites. Macroscopically, the granulite–eclogite interface is sharp and there are no significant compositional changes in the bulk chemistry, indicating the fluid composition was quickly rock buffered. To better understand the link between deformation, fluid influx, and fluid–rock interaction one cm-wide shear zone at incipient eclogitization is studied here. Granulite and eclogite consist of garnet, pyroxene, and plagioclase. These nominally anhydrous minerals (NAMs) can incorporate H2O in the form of OH groups. H2O contents increase from granulite to eclogite, as documented in garnet from ~ 10 to ~ 50 µg/g H2O, pyroxene from ~ 50 to ~ 310 µg/g H2O, and granulitic plagioclase from ~ 10 to ~ 140 µg/g H2O. Bowl-shape profiles are characteristic for garnet and pyroxene with lower H2O contents in grain cores and higher at the rims, which suggest a prograde water influx into the NAMs. Omphacite displays a H2O content range from ~ 150 to 425 µg/g depending on the amount of hydrous phases surrounding the grain. The granulitic plagioclase first separates into a hydrous, more albite-rich plagioclase and isolated clinozoisite before being replaced by new fine-grained phases like clinozoisite, kyanite and quartz during ongoing fluid infiltration. Results indicate a twofold fluid influx with different mechanisms to act simultaneously at different scales and rates. Fast and more pervasive proton diffusion is recorded by NAMs that retain the major element composition of the granulite-facies equilibration where hydrogen decorates pre-existing defects in the crystal lattice and leads to OH increase. Contemporaneously, slower grain boundary-assisted aqueous fluid influx enables element transfer and results in progressive formation of new minerals, e.g., hydrous phases. Both mechanisms lead to bulk H2O increase from ~ 450 to ~ 2500 µg/g H2O towards the shear zone and convert the system from rigid to weak. The incorporation of OH groups reduces the activation energy for creep, promotes formation of smaller grain sizes (phase separation of plagioclase), and synkinematic metamorphic mineral reactions. These processes are part of the transient weakening, which enhance the sensitivity of the rock to deform.

Numerical Simulations Reproduce Field Observations Showing Transient Weakening During Shear Zone Formation by Diffusional Hydrogen Influx and H2O Inflow

“…The following descriptions and interpretations are mainly based on the analysis of Kaatz et al. (2022) but are similar to those described by various studies on Holsnøy (e.g., Austrheim, 1987; Boundy et al., 1992; Jamtveit et al., 1990; Putnis, 2021; Zertani et al., 2019).…”

“…However, within the scope of a previous study, Kaatz et al. (2022) estimated a duration of 2 years for the hydration of a single grt grain within the same investigated shear zone. This duration reflects the time the grt had to equilibrate with the elevated hydrogen concentration during the fluid‐mediated shear zone formation.…”

“…Contemporaneously, the slower grain boundary‐assisted inflow of aqueous fluid enables element transfer and exchange by dissolution‐precipitation mechanisms (e.g., Putnis & John, 2010; Zertani et al., 2022). Hence, the addressed microstructural transformations occur while new eclogite‐facies and hydrous minerals form and incorporate OH‐groups (e.g., Kaatz et al., 2022; Mukai et al., 2014; Petley‐Ragan et al., 2018; Zertani et al., 2019). One aim of this study was to gain new insights into whether a preweakening caused by a hydrogen influx is feasible and how it affects the subsequent inflow of an aqueous fluid.…”

“…Furthermore, studies have shown that hydrogen in the crystal lattice reduces the activation energy of creep mechanisms, especially climb, for example, in olivine (e.g., De Ronde et al., 2005; Mackwell & Kohlstedt, 1990; Mackwell et al., 1990). At the Holsnøy site, these effects may have contributed to the transient weakening and shear zone formation (e.g., Herwegh et al., 2008; Kaatz et al., 2022). In general, plg is the most abundant phase in crustal rocks and, hence, determines their mechanical strength and deformability (e.g., Behrens, 2021; Mosenfelder et al., 2015; Rybacki & Dresen, 2000, 2004).…”

“…Importantly, Kaatz et al. (2022) showed that plg breaks down after limited hydrogen incorporation, causing a matrix grain‐size minimization along with the formation of hydrated plg and clinozoisite (czs). Furthermore, studies have shown that hydrogen in the crystal lattice reduces the activation energy of creep mechanisms, especially climb, for example, in olivine (e.g., De Ronde et al., 2005; Mackwell & Kohlstedt, 1990; Mackwell et al., 1990).…”